1. Field of the Invention
The present invention relates to an image pickup apparatus and a control method that correct image data taken by an image pickup device.
2. Description of the Related Art
Conventionally, it is well known that in image pickup apparatuses, when exposure is performed for a long period of time, shooting is performed at high speed, or shooting is performed at high temperature using a solid-state image pickup device (hereafter referred to as an image pickup device), dark current develops in the image pickup device.
The reason why dark current develops in the image pickup device is that the image pickup device has a property of not only converting light energy into electric signals but also converting thermal energy into electric signals. It is known that dark current is heavily dependent on temperatures, and when the temperature rises by 8° C. to 10° C., the output of dark current almost doubles. Because dark current developing in the image pickup device adversely affects image quality, various measures have been taken in terms of manufacturing so as to decrease dark current components.
Moreover, not only when the temperature of an image pickup device as a whole rises due to an installation environment in which the image pickup device is mounted in an image pickup apparatus or the like, but also when the amount of electric current consumed by a part of the image pickup device (for example, the amount of electric current consumed by an amplifier installed in an output stage is large), a phenomenon described hereafter may occur. Part of the image pickup device may increase in temperature, and dark current in only this part increases, resulting in an increase in the output of the image pickup device. For example, when exposure is performed for a long period of time in high-speed shooting using an image pickup apparatus (that is, when a night scene or the like is taken), a light of color close to magenta may appear in a region of an image taken by an image pickup device where there is no light under normal conditions.
There has been the problem that when dark current of an image pickup device increases as described above, this greatly affects image quality, and for example, in-surface brightness and color balance of the image pickup device go awry when the in-surface distribution of the dark current in the image pickup device is uneven. As a process for addressing such a problem, a noise reduction process (so-called dark image subtraction process) is known. The dark image subtraction process is a process in which after an original image as a target to be shot is taken, an image is taken (dark image) with an image pickup device shielded from light under the same conditions as those when the original image was taken, and long exposure is performed so as to cancel effects of dark current by subtracting the dark image from the original image.
However, the dark image subtraction process generally has problems described hereafter.
To reliably cancel the effects of dark current when an original image is taken, it is preferred that a dark image is obtained under the same conditions as those when taking the original image was taken. For this reason, when long exposure is performed, a dark image obtainment time period equal to the exposure time period is needed, and hence operability of an image pickup apparatus deteriorates. Moreover, when a dark image is subtracted from an original image, random noise which the original image and the dark image have is √2 times random noise which the original image has, and hence substantial degradation of image quality is unavoidable.
For this reason, a method has been proposed in which in order to dispense with the dark image subtraction process when possible when shooting conditions do not require it, dark current components are detected when an original image is taken, and when the detected dark current components are more than a predetermined amount, the image subtraction process is performed.
On the other hand, as one type of pixel abnormalities of an image pickup device, there is one called a white defect. The white defect is a pixel that appears as a white spot when an image is formed when the output of dark current is very large due to a factor such as temperature or exposure time because the amount of dark current in the pixel is abnormally larger than in other pixels.
The white defect is corrected for as described below because addresses at which white defects appear and levels of white defects differ according to image pickup devices. Specifically, addresses at which white defects appear are identified in a manufacturing step for the image pickup device, and an address and a defect level are detected and stored, and pixels having abnormal output values are extracted from a taken image, thus identifying white-defect pixels. Further, a white defect pixel is corrected for by interpolating from peripheral pixels.
As for white defects, there is the problem that conspicuous signs of correction appear in correction pixels when thin lines are shot, and hence there has been proposed that, whether or not to correct for white defects is determined according to the amount of dark current so as to dispense with unnecessary correction.
As a method to detect dark current, a technique described hereafter has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-158626). According to the technique proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2007-158626, a black reference pixel region (null pixels) that does not have a photoelectrical conversion element structure and a black reference pixel region (OB (optical black) pixels) that has a photoelectrical conversion element structure but is shielded from light are disposed in a pixel region of an image pickup device. Thus, an output value of a null pixel including no unnecessary dark current can be subtracted from an output value of an OB pixels including dark current.
Also, as a technique related to the present invention, described later, a technique described hereafter has been proposed (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2005-175930). According to the technique proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2005-175930, an image pickup device has at least two regions (OB1 and OB2) having photoelectrical conversion elements of different sizes, and an OB pulse is selected by storing a relationship between shooting conditions and environmental (temperature) conditions and OB pulses in a control circuit or memory of the image pickup device in advance. Thus, output differences from an effective pixel unit are corrected for. However, there is the problem that when the temperature of the image pickup device cannot be accurately detected due to a failure in a temperature detection circuit or the like, desired results cannot be obtained if control is provided using OB pulses stored in advance.
Moreover, there has been proposed a technique to obtain output information from both an OB unit and a simulated black level pixel unit and perform predetermined image processing (black level adjustment) based on the obtained output information (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2007-27845). According to the technique proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2007-27845, a process to cope with a case where output information from both the OB unit and the simulated black level pixel unit cannot be accurately measured is not disclosed. Also, although it is described that processing contents are changed according to temperature, there is the problem that a desired process cannot be performed when an external temperature sensor fails because temperature information is obtained from the external temperature sensor.
Referring now to FIGS. 16A to 16C, a brief description will be given of an image pickup device. FIG. 16A shows an exemplary pixel arrangement of the image pickup device, which has an effective pixel unit 1001, an OB pixel unit 1002, and a null pixel unit 1003. FIG. 16B shows pixel output in a dotted line region in FIG. 16A when the amount of dark current is small and in a case where difference computation is possible (OB pixel output−null pixel output: measurable). FIG. 16C shows pixel output in the dotted line region in FIG. 16A when the amount of dark current is large and in a case where difference computation is impossible (OB pixel output−null pixel output: unmeasurable).
In general, in an image pickup apparatus having the image pickup device shown in FIGS. 16A to 16C, to prevent dark current output from compressing a computation dynamic range of a subject image during shooting, clamping is performed so that output from the OB pixel unit 1002 can be inside a predetermined output range. In this case, the predetermined output range is generally determined so as to be in a direction opposite to a light output direction in the computation dynamic range and to be inside the computation dynamic range. In the image pickup device, a dark current output direction and the light output direction are the same.
In the image pickup device shown in FIGS. 16A to 16C, when the amount of dark current is large as in shooting at high temperature, shooting at high speed, or shooting involving long exposure, output from the OB pixel unit is fixed at a clamping level irrespective of the amount of dark current, but there is a great difference in output between the OB pixel unit and the null pixel unit. In the example shown in FIG. 16C, because output from the null pixel unit exceeds the computation dynamic range, differences in output between the OB pixel unit and the null pixel unit cannot be accurately measured. Also, due to a large amount of dark current, a dark current value cannot be accurately obtained. Thus, there is the problem that the dark image subtraction process and the white defect correction process cannot be performed under desired dark current conditions.